The present invention relates to optical devices with diffraction gratings; more particularly, the present invention relates to functional devices for multi-wavelength signal manipulation in fiber-optic networks that are based on focusing and dispersive properties of concave diffraction gratings.
Fiber optics has become the core of telecommunication and data networking infrastructures. Optical amplification, routing and switching for wavelength division multiplexing and multi-wavelength channels are the comer stones of current and future fiber networks. Low cost and efficient technology is needed for manufacturing of optical components for WDM fiber optic networks.
U.S. Pat. No. 5,414,540, entitled xe2x80x9cFrequency-selective optical switch employing a frequency dispersive element, polarization dispersive element, and polarization modulating element,xe2x80x9d issued May 9, 1995, describes a frequency-selective optical switch employing a frequency dispersive element, polarization dispersive element, and polarization modulating element. The same scheme is proposed for an add-drop multipexer implementation. The disclosed switch uses diffraction gratings, collimating objectives, birefringence crystals, and a liquid crystal cell array. The design is complicated and very sensitive to alignment tolerances.
A free space version of multi-channel wavelength attenuator was described in J. E. Ford, et al., xe2x80x9c32 Channel WDM Graphic Equalizerxe2x80x9d in IEEE/LEOS 1996 Summer Topical Mtg Digest, pp. 26-27. FIG. 10 represents an optomechanical design of 32-channel WDM equalizer from the article. In this design, the fiber input is collimated by a doublet lens, diffracted by a blazed grating, focused by a triplet lens on a modulator, and reflected light is focused one more lens into output fiber. Overall efficiency is limited by multiple surface reflections.
Multiplexers and demultiplexors accommodating multi-wavelengths have been based on planar waveguide technology. Such implementations usually include techniques to flatten the passband. There are few reasons why the standard Gaussian shape of passband is not preferred for practical implementation. First, the transmitting laser wavelength can drift away from the passband peak due to temperature variation, for example. Thus, there will be additional loss if the passband is Gaussian. If multiple multiplexers and demultiplexers are cascaded along the transmission line, the overall throughput is reduced when the transmission bands have slightly different center wavelengths due to manufacturing tolerances or temperature variations. Even if the bandwidth centers coincide, the resulting passband width is much narrower due to the Gaussian passband shape. The flat top passband shape is preferred, since it doesn""t have such unwanted properties. Methods to flatten the passband shape are already known in planar waveguide technology. For example, an MMI coupler input waveguide or parabolic waveguide horn design have been used in planar multiplexer/demultiplexer. See K. Okamoto, xe2x80x9cPlanar Lightwave Circuits (PLC)xe2x80x9d Photonic Networks-advances in Optical Communication, Springer-Verlag, 1997 pp. 118-132 and K. Okamoto and A. Sugita, xe2x80x9cFlat Spectral Response Arrayed-Waveguide Grating Multiplexer with Parabolic Waveguide Horn,xe2x80x9d Electron. Lett., Vol. 32, pp. 1612-1661. U.S. Pat. No. 4,387,955 issued Jun. 14, 1983 entitled xe2x80x9cHolographic Reflective Grating Multiplexer/Demultiplexer,xe2x80x9d describes a Multiplexer/Demultiplexer based on a holographically recorded concave grating in which an input signal having multiple wavelengths is separated into multiple signals, each being of a different one of the wavelengths.
An optical device is described. In one embodiment, the optical device includes first and second concave diffraction gratings, first and second concentrators, first and second sets of fibers, and an array of optical elements. The first concave diffraction grating reflects light from an input beam into multiple light waves of different wavelengths focused at spatially separate locations. The first concentrator has a first set of light guiding channels to transfer the light waves. The first set of fibers is optically coupled to the first set of light guiding channels. The array of optical elements include s optical elements that have at least one input and at least one output. Such inputs of each optical element in the array is coupled to one of the first set of light guiding channels. The second set of fibers is optically coupled to outputs of optical elements in the array. The second concentrator has a second set of light guiding channels. Each of the second set of light guiding channels is optically coupled to a distinct fiber in the second set of fibers. The second concave diffraction grating reflects light from the second concentrator into a single light beam focused at an output fiber.